[Patch] (general Fortran + OpenMP) [Fortran] Fix/modify present() handling for assumed-shape optional (PR 94672)

[Tobias Burnus]

The main purpose of this patch is to fix OpenMP, but it modifies
the general Fortran handling of assumed-shape optional arguments.

For assumed shape, gfortran generates an "arg.0 = arg->data"
artificial variable – and with optional one has something like

if (arg != NULL && arg->data != NULL)
   {
     arg.0 = arg->data;
     lbound.0 = ...
   }

And an "if (present(arg))" becomes
"if (arg != NULL && arg->data != NULL)".

The proposed change changes the init to:

if (arg != NULL && arg->data != NULL)
   {
     arg.0 = arg->data;
     lbound.0 = ...
   }
else
   arg.0 = NULL;  // <-- new

Such that an "if (present(arg))" becomes "if (arg.0 != NULL)".

I think for Fortran code itself, it does not really make any
difference. However, for OpenMP (and OpenACC) it does.

Currently,
   !$omp …
     if (present(arg)) stop 1
   !$omp end …

has decl = "arg.0" and SAVED_DESCRIPTOR(decl) == "arg" such
that inside the omp block everything is "arg.0" – except for
"if (present(arg))" which is converted to the "!arg && !arg->data".

This causes the problems shown in the PR (PR94672).

For optional & 'omp target' where one has to map the variable and
has to check it inside the target function, I even ended up setting
"arg.0 = NULL" explicitly as this was much simpler than adding more
checking in gcc/omp-low.c.


Thus: I think either variant (checking arg directly vs. checking arg.0
plus setting it to NULL) works equally well with normal Fortran code;
one can probably design code where one or the other is slightly faster,
but at the end it should not matter.
And for OpenMP/OpenACC, the new variant avoids several problems.

Hence:
OK for the trunk – and GCC 10 (regression, rejects valid code)?

Tobias

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